Photoconductive layers for electrophotography



Nov. 2, 1965 sc M ETAL 3,215,528

PHOTOGONDUCTIVE LAYERS FOR ELECTROPHOTOGRAPHY Filed Nov. 2, 1962INVENTORS GUSTAV SCHAUM, H/LDEGAPD HAYDEN, AN/M VON KON/G, HEINRICHSE/BERT.

BY CM 463.1. ATTO NE United States Patent Office 3,215,523 Patented Nov.2, 1965 3,215,528 PHOTOCONDUCTEVE LAYERS FOR ELECTROPHOTOGRAPHY GustavSchaurn, Hildegard Haydn, Anita von Kiinig, and Heinrich Seibert,Leverkusen, Germany, assignors to Agfa Aktiengesellschaft, Leverkusen,Germany, a corporation of Germany Filed Nov. 2, 1962, Ser. No. 234,926Claims priority, application Germany, Nov. 14, 1956, A 26,007 9 Claims.(Cl. 96-1) This application is a continuation-in-part of our applicationSerial No. 696,032, filed November 13, 1957, now abandoned.

The present invention relates to photoconductive layers forelectrophotography.

For the production of photoconductive layers for eletrophotography it isknown to use certain inorganic or organic photoconductive materials.Examples of such materials are sulphur, selenium, oxides, sulphides andselenides of zinc, cadmium, mercury, antimony, bismuth, and lead, andalso anthracene and anthraquinone. These substances are applied to anelectrophotographic plate, especially a plate consisting of metal oranother material, for example paper, or to a sheet or foil produced froma fihn-forming plastic. If necessary, the photoconductive substance canbe held dispersed in an electrically isolating, layer-forming binder.Such layers being produced by means of coating solutions in which thebinders are either dissolved or dispersed and in which theelectro-conductive substances are dispersed. Such photoconductive layersare for instance disclosed in United States Patents No. 2,297,691;2,357,809; 2,727,807; 2,727,808; 2,735,784; and 2,735,785, and in FrenchPatents No. 1,113,933; 1,122,275; 1,125,235; and 1,136,146.

It has now been found that electrophotographic layers with excellentproperties and which are only slightly colored are obtained ifpolynuclear aryl compounds which contain at least 2 benzene ringsconnected to each other by a single carbon-to-carbon bond and which aresubstituted by at least 1 acyl or substituted acyl radical are used asphotoconductive compounds in such electrophotographic layers.

The following compounds are, for example, suitable for the instantprocess:

M.P. 4-b enzoyl-p-terphenyl 214 4 4-chlorobenzoyl) -p-terphenyl 2 69-704 (2,5-dichlorobenzoyl p-tertphenyl 184-85 4-acetyl-p-terphenyl 200-2014,4"-diacetyl-p-pterphenyl 283-84 4,4"dibenzoyl-p-terphenyl 294-95 4,4di(4-chlorobenzoyl) p-terphenyl 300 4(p-phenylbenzoyl)p-terphenyl 299-3004,4-di-(2,5-dichlorobenzoyl)-p-terphenyl 241-42 The method for thepreparation of 4-benzoyl-p-terphenyl is described in J. Org. Chem. 19,730 (1954). The other compounds are prepared accordingly by the reactionof an acylchloride with terphenyl in the presence of a Fn'edel-Craftscatalyst such as AlCl Suitable acyl groups are, for example, loweraliphatic acyl groups or benzoyl, the benzene rings of which can befurther substituted, for example, by halogen, chlorine or bromine or byfurther phenyl rings.

The photoconductive compounds according to the invention areadvantageous over prior art because they are more sensitive to longerwavelength. It is furthermore possible to sensitize these substances forstill longer wavelengths by suitable compounds as they are for instancedisclosed in French Patent No. 1,125,235.

The coating solution which is used for the production of thephotoconductive layers comprises at least one of the aforementionedsubstances and a film-forming binding agent, the electrical specificresistance of which must be higher than that of the photoconductivesubstance and also higher than that of the layer support. The bestresults are obtained with binding agents the electrical specificresistance of which amounts to at least 10 ohm-centimeters.

Particularly suitable binding agents are silicone resins such asalkylpolysiloxanes and arylpolysiloxanes, especially phenyl and methylpolysiloxanes as they are disclosed in German Patents No. 853,351 and865,975.

Further suitable binding agents are for instance cellulose, celluloseesters, cellulose ethers, polyvinyl chlo ride, polyurethanes,polyesters, polyamides, polycarbonates with a base ofdi-(monohydroxyaryl)-alkanes, especially4,4'-di(m0nohydroxyaryl)-alkanes according to German Patents No. 971,790and 971,777. Photoconductive compounds can be present in theelectrophotographic layers, homogeneously in dissolved form orheterogeneously in a suspended form whereby the particle size of thesuspended photoconductor should be as small as possible.

The quantitative ratios between the photoconductive substances and thebinding agents may vary within wide limits. It is preferred to use thephotoconductive substance in amounts between 1 part for each 0.3 to 2parts by weight of binding agent which is the equivalent of 0.5 to 3.33parts by weight of the photoconductive substance to each part by weightof the binding agent, and amounts between about 5 and about 40 grams persquare meter of photoconductive layer. The solvent or solvent mixtureused for the production of electrophotographic layers containing aphotoconductive compound, heterogeneously dispersed therein must be goodsolvents for the binding agent but non-solvents or poor solvents for thephotoconductive compounds. Suitable layers of this type can be producedas follows: An organic solvent is used which dissolves both thephotoconductive substance and the binding agent layer former. Thissolution has added thereto another organic solvent in which the layerformer is soluble but the photoconductive substance is insoluble. Bythis means, the photoconductive compound is deposited in a state ofparticularly fine distribution, so that layers with a particularlysmooth surface are obtained. As a support for the photoconductive layerthere may be used paper or metal plates, such as zinc, aluminum, orbrass plates. Furthermore, thin foils of cellulose hydrate, celluloseesters or of polyamide come into question. In processing of theelectrophotographic materials said material is electrostatically chargedin the dark, for instance, by meaning of a corona discharge device. Thecharged layer is then exposed with light through a nega tivephotographic film, positive film or mask or otherwise to a light image,to receive a latent electrostatic image. If the electrophotographicmaterial has been properly prepared, the charges leak 01? rapidly to thesupport in proportion to the intensity of light to which any given areais exposed. After such exposure, the latent electrostatic image can bedeveloped by any developing process known per se, for example, bydusting a developing powder whereby the powder adheres to the areaswhere the electrostatic charges remain, forming thereby a powder imagecorresponding to the electrostatic image. Thereafter the powder imagecan be fixed by melting the developing powder or can be transferred to asheet of transfer material resulting in a positive or negative print asthe case may be.

The photoconductive layers can also be produced from aqueous dispersionsof photoconductive substances and binding agents, for instance by theprocess disclosed in French Patent No. 1,136,146. If necessary, the castdispersions or emulsions are subjected to a final condensation or finalpolymerization by heat treatment at temperatures of about 80-150 C. forabout 2 to 30 minutes after being dried. Aqueous dispersions ofpolymeric substances, such as melamineformaldehyde or urea-formaldehyderesins, xylene-formaldehyde resins, polymers based on vinyl chloride,vinylidene chloride, vinyl ethers, acrylic esters, methacrylic esters,acrylic amides, such for example as methacrylic amide, aromatic vinylcompounds, such for example as styrene, isoolefins, such for example asisobutylene, copolymers based on the aforementioned compounds, such forexample as copolymers of vinyl chloride and butyl acrylate vinylidenechloride and butyl acrylate synthetic elastorners, such for example ascopolymers of butadiene and styrene, butadiene and acrylonitrile, andalso copolymers of dienes with a preponderant proportion of styrene and/or acrylonitrile or other vinyl compounds, as well as polyamides,polyurethanes, polyesters of polycarboxylic acids and polyhydricalcohols, polycarbonates, cellulose ester, rubber and the like, are forexample employed.

Example 1 40 g. of 4,4-"-diacetyl-p-terphenyl,

64 g. of a 60% solution of a silicon resin in toluene, for instance asilicone resin according to Example 1, German Patent No. 853,351,

97 cc. toluene are thoroughly mixed and ground for several hours in aball mill.

The resulting mixture is then cast on a paper support and dried.

By usual processing which comprises charging by corona dischargeexposing and developing the resulting electrostatic latent image, adeveloped powder image is obtained having excellent detail and quality.

Example 2 30 g. of 4,4"-di(2,5-dichlorobenzoyl)-p-terphenyl are added toa mixture of 100 cc. of a 30% aqueous dispersion of a heat-hardenablemelamine-formaldehyde condensation product and 80 cc. of water. Themixture is treated for 2 hours in a ball mill and thereafter coated on apaper support. After drying at room temperature the material is heatedto 80 C. for minutes.

By usual processing which comprises charging by corona dischargeexposing and developing the resulting electrostatic latent image, adeveloped powder image is obtained having excellent detail and quality.

Example 3 In this example the electrophotographic properties of severalcompounds according to the invention are compared with knownphotoconductive compounds.

The sensitivity of a electrophotographic material depends essentially onthe difference between the photoconductivity and the dark-conductivity.The more electrostatic charge drains off by a given exposure the higheris the contrast and the quality of the resulting image. The graphsattached thereto present the draining off of the charge by exposure as afunction of time. The steeper the slope of the curve the more suitableis the compound for electro-photographic purposes. The axis of abscissashows the relative charge per unit area in logarithmic scale inarbitrary units and the axis of ordinate the time in arbitrary units.

The electrophotographic material to be tested where prepared accordingto Example 2 by mixing each 40 g. of photoconductive compounds with 64g. of a solvent of the silicone resin described in that example. Witheach compound the processing, charging and exposure used identicaltechnique.

The curves in FIGURES 1 and 2 represent the following photoconductivecompounds:

FIGURE 1: Known photoconductive compounds. Curve 1: anthracene Curve 2:p-terphenyl Curve 3: anthraquinone FIGURE 2: Photoconductors accordingto the invention. Curve 1: 4,4"-dibenzoyl-p-terphenyl Curve 2:4,4"di-(4-chlorobenzoyl)-p-terphenyl.

We claim:

1. An electrotophotographic material comprising a support, and aphotoconducting insulating layer coated thereon composed of a dielectricfilm forming organic resin and a photoconductive organic compounddispersed therein, said organic compound being a polynuclear arylcompound which contains between 2 and 3 benzene rings connected to eachother by single carbon to carbon bonds and substituted by at least 1acyl radical.

2. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compound4,4"-diacetyl-p-terphenyl.

3. An electrophotographic material according to claim 1,-wl1erein saidphotoconducting insulating layer contains as a photoconductive compound4,4"-di-(2,5-dichlorobenzoyl -p-terphenyl.

4. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compound4,4-di-(4-chlorobenzoyl)- p-terphenyl.

5. In an electrophotographic reproduction process which comprisesexposing an electrostatically charged supported photoconductiveinsulating layer to light under a master to discharge the layer inproportion to the intensity of light to which any given area is exposedand developing the resulting latent electrostatic image with anelectrophotographic developer, the improvement according to which thephotoconductive insulating layer has as photoconductive ingredient apolynuclear aryl compound which contains between 2 and 3 benzene ringsconnected to each other by single carbon to carbon bonds and substitutedby at least 1 acyl radical. I

6. The combination of claim 5, wherein said photoconductive layercontains as a photoconductive compound 4,4"-diacetyl-p-terphenyl.

7. The combination of claim 5, wherein said photoconductive layercontains as a photoconductive compound4,4"-di-(2,5-dichlorobenzoyl)-p-terphenyl.

8. The combination of claim 5, wherein said photoconductive layercontains as a photoconductive compound 4, 4"-di (4-chl orobenzoyl-p-terphenyl.

9. The combination of claim 1 in which the polynuclear aryl compound ispresent in a concentration of between about 5 and 40 grams per squaremeter of photoconductive layer, and between about 0.5 to 3.33 parts byweight to each part of the resin by weight.

No references cited NORMAN G. TORCHIN, Primary Examiner.

1. AN ELECTROTOPHOTOGRAPHIC MATERIAL COMPRISING A SUPPORT, AND A PHOTOCONDUCTING INSULATING LAYER COATED THEREON COMPOSED OF A DIELECTRIC FILM FORMING ORGANIC RESIN AND A PHOTOCONDUCTIVE ORGANIC COMPOUND DISPERSED THEREIN, SAID ORGANIC COMPOUND BEING A POLYNUCLEAR ARYL COMPOUND WHICH CONTAINS BETWEEN 2 AND 3 BENZENE RINGS CONNECTED TO EACH OTHER BY SINGEL CARBON TO CARBON BONDS AND SUBSTITUTED BY AT LEAST 1 ACYL RADICAL. 